Soap is made by the alkaline hydrolysis of vegetable oils and animal fats. The fat or oil is heated with concentrated aqueous sodium hydroxide. Each of the three ester groups in the triglyceride is hydrolysed, giving propane-1,2,3-triol (glycerol) and the sodium salts of the long-chain carboxylic acids. These salts are soap.triglyceride + 3NaOH → CH2(OH)CH(OH)CH2OH + 3RCOO−Na+ For example, if every chain is C17H35, the soap is sodium octadecanoate (sodium stearate), C17H35COO−Na+. Using potassium hydroxide instead gives potassium salts, which make softer soaps.How soap cleans. A soap anion has an ionic COO− head, which is attracted to water, and a long hydrocarbon tail, which dissolves in grease and oil. The tails bury themselves in a grease droplet while the heads stay in the water, so the grease is lifted off and washed away.
Worked example: how many moles of NaOH are needed to saponify 0.050 mol of a triglyceride? Each triglyceride has three ester groups, so the ratio is 1 : 3. Moles of NaOH = 3 × 0.050 = 0.15 mol. The same reaction also forms 0.050 mol of glycerol, a useful by-product used in cosmetics and pharmaceuticals.
Worked example: how many moles of NaOH are needed to saponify 0.050 mol of a triglyceride? Each triglyceride has three ester groups, so the ratio is 1 : 3. Moles of NaOH = 3 × 0.050 = 0.15 mol. The same reaction also forms 0.050 mol of glycerol, a useful by-product used in cosmetics and pharmaceuticals.